1. Related Application
This application is related to Applicant's copending application Ser. No. 133,546, filed Oct. 2, 1993, PHN 14,285, assigned to the same assignee.
2. Field of the Invention
The present invention relates to a direct conversion receiver comprising a local frequency generating arrangement which is coupled to a pair of quadrature related mixers for mixing down an rf input signal to quadrature related signals, and a.f.c. means for providing a control signal for the local frequency generating arrangement, the a.f.c. means, which are coupled to quadrature paths, comprising a frequency offset detector for deriving a frequency control signal having an average value which is proportional to the frequency offset of the local frequency generating arrangement with respect to the rf input signal. The frequency offset detector comprises a series arrangement of a phase shifting arrangement and a multiplication arrangement in a quadrature path. Such direct conversion, receivers can be digital paging receivers or transceivers using an FSK (Frequency Shift Keying) modulation scheme, but also transceivers for cordless telephony or the like.
3. Description of the Related Art
A direct conversion receiver of this kind is known from the European Patent EP 298 484 B 1. In the known direct conversion receiver, which is an optical receiver, an a.f.c. (automatic frequency control) signal for a local oscillator with inherent correct polarity is derived from so-called I- and Q-channel receiver paths. For this purpose, a baseband I-channel signal is fed to a frequency offset detector, together with a quadrature related Q-channel signal. The frequency offset detector is a series arrangement of a phase shifting arrangement and a multiplication arrangement, whereby the I-channel signal is fed to an input of the multiplication arrangement, and the Q-channel signal is fed to another input of the multiplication arrangement via the phase shifting arrangement. The output signal of the detector is used as a control signal which is fed to the local oscillator via a low pass filter. In the known receiver the control signal is a bipolar signal, which is subject to a relatively strong low frequency ripple component.